PERTH (AFP) - Malaysian Prime Minister Najib Razak is to tour the military base being used as a staging post in the multinational hunt for Flight MH370 on Thursday as Australia said there are no imminent plans to abandon the search.

Kuala Lumpur's handling of the crisis sparked by the disappearance of a Malaysia Airlines jet carrying 239 people on March 8 has been widely criticised, especially by distraught relatives of the 153 Chinese nationals aboard.

In contrast, Australia's mobilisation since it was handed increased responsibility in the search effort has been praised and Prime Minister Tony Abbott said he was proud of the way the country had handled the situation.

"Provided we continue to conduct this search and recovery exercise with the kind of professionalism, dedication and intensity that we have had over the last three weeks or so, I think it has got to be good for our reputation," he told ABC radio.

Australia has far more experience than Malaysia of search and rescue operations, routinely monitoring huge tracts of ocean, and Razak will tour the Pearce air base near Perth to see the eight-nation operation first hand and meet those involved.

Despite extensive scouring of the remote southern Indian Ocean off Perth where Malaysia believes the jet went down, no debris that would indicate a crash site has so far been found.

Adding to the frustration for relatives of those on board, Malaysian police chief Khalid Abu Bakar said on Wednesday a criminal investigation into what caused the flight to veer far from its intended route between Kuala Lumpur and Beijing had so far been inconclusive.

Mr Abbott, who will accompany Mr Najib on Thursday, said the exhaustive quest for answers would continue, although he cautioned that it could not go on forever.

"Obviously, at some point we would decide that we have recovered whatever we are going to recover from the ocean but that point is, as yet, quite a long way off," he said.

A British nuclear submarine with underwater search capabilities reinforced the hunt on Wednesday, joining planes and ships scouring 237,000 square kilometres (91,500 square miles) of desolate seas, but they again reported seeing nothing of interest.

Australia's Joint Agency Coordination Centre, which is handling the search effort, said eight planes and nine ships would be involved Thursday as they further refined the area they are looking at to 1,680 kilometres west north-west of Perth.

The weather is forecast to be good, with visibility of 10 kilometres.

Australia's Ocean Shield naval vessel, which is fitted with a US-supplied "black box" detector, is expected to arrive in the area Friday, but it it faces a daunting task and the search zone needs to be narrowed for it to have any hope of finding evidence of the doomed jet.

Malaysian police chief Khalid on Wednesday pleaded for more time to work out what had happened, with news agency Bernama quoting him as saying all passengers had been "cleared" of suspicion.

Police are investigating the backgrounds of the plane's 12 crew, as well as ground staff and flight engineers, for signs of a hijack or sabotage plot.

Nearly a month after the flight's disappearance, authorities remain baffled as to how and why it happened, and warn that unless the black box is found, the mystery may never be solved.

The battery-powered signal from MH370's black box - which records flight data and cockpit voice communications - usually lasts only about 30 days, with time fast running out.

In a briefing on Wednesday, Malaysian officials sought to explain to sceptical relatives their conclusion that the plane went down in the Indian Ocean.

That determination, based on complex satellite data, has outraged some families who say wreckage must first be recovered.

The closed-door briefing outside Kuala Lumpur included technical experts from China and Malaysia, a government statement said, and was telecast to other relatives in Beijing.

"Throughout the process, I want to assure you that we have done everything in our power to locate MH370," civil aviation chief Azharuddin Abdul Rahman told relatives.

But Steven Wang, a relative in Beijing, said he was unconvinced.

"I don't think that anything was explained to us.... There are all kinds of possibilities," he said.

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